Spectrometers have gained popularity as a tool for measuring attributes of tissue. By way of illustration only, the operation of an instrument of this type is described briefly with reference to prior art FIG. 1. As shown, the instrument 10 included an optical probe 12 which was releasably connected to an electronics package 14 via optical fibers 16. The electronics package 14 included a connector 18, a detector 20, a processor/controller 22, and a display 24. In operation, the probe 12 was positioned on the tissue to be measured or analyzed. The probe 12 was interfaced to the instrument electronics through the optical fibers 16 and a probe connector 26. Referring now to prior art FIG. 2, the probe connector 26 included light emitting diodes (LEDs) or other light sources 30, 32, 34, 36, and 38 for generating light at a number of different wavelengths (e.g., 800, 760, 720, 680, and 530 nm, respectively). The light used to measure the characteristics of the tissue was coupled to the probe 12 by send optical fibers 40, 42, 44, and 46. After being transmitted from the tissue-engaging surface of the probe 12 into the tissue being measured, the light traveled through the tissue before being collected at the end of the receive optical fiber 48. This collected light (measurement light signal) was then transmitted to the instrument 14 through the probe connector 26 and electronics package connector 18. A reference light signal corresponding to each of the measurement light signals (i.e., the reference light signals were not transmitted through the tissue) was also transmitted to the electronics package connector 18. The optical probe 12 is described in greater detail in Provisional U.S. Patent Application Ser. No. 60/137,383 entitled “Disposable Tissue Probe Tip” and U.S. Pat. No. 6,487,343 entitled “Fiber Optic Light Mixer.”
The collected measurement light signals and reference light signals received by the electronics package 14 were transmitted to the detector 20 which produced electrical signals representative of these light signals at each wavelength of interest. The processor/controller 22 then processed these signals to generate data representative of the measured tissue parameter (e.g., saturated oxygen level (StO2)). The measurement reading could have been visually displayed on the display 24. Algorithms used to compute the tissue parameter data are generally known and described in U.S. Pat. No. 5,879,294 entitled “Tissue Chromophore Measurement System.”
Calibration procedures were typically performed to enhance the accuracy of the measurements subsequently made by the instrument 14. Methods and devices for calibrating spectrophotometric-type instruments are generally known and disclosed in the above-referenced U.S. patent entitled “Tissue Chromophore Measurement System.” The calibration could have, for example, been performed by placing the probe 12 on a calibration device 50 such as that shown in FIG. 1. The calibration device 50 included a housing which was filled with light scattering material. The light scattering material was generally spectrally flat (i.e., reflects all light to the same degree) to provide a reference spectrum. White polyethylene foam such as Plastazote LD45 available from Zotefoams, Inc. could have been used for this purpose.
One configuration of a spectrophotometric instrument of the type described above included, for each wavelength of interest, a photomultiplier tube (PMT) for detecting the measurement light signal, and a photodiode for detecting the calibration recognition signal (or ambient light). Thermal electric coolers could have been included in the electronics package to help maintain temperature control of the optical bench to which the PMTs and photodiodes were mounted, and thereby reduce output signal drift.
The probe connector 26 used in connection with this device is illustrated in prior art FIG. 2, which shows an embodiment having a reference signal generated within the connector 26. As shown, the probe connector 26 included 4 LED's 30, 32, 34, and 36 for generating the measurement light signals at 800, 760, 720 and 680 nm. Light signals from each of these LEDs were coupled to the probe 12 by a separate measurement signal send fiber 40, 42, 44, 46. After being transmitted through the tissue being analyzed and collected at the probe, the measurement light signal was coupled back to the probe connector 26 by a measurement signal receive fiber 48. The end of the measurement signal receive fiber 48 terminated in the probe connector 26 at a sample ferrule 52 which was adapted to mate with a socket in the connector 18 of the electronics package 14. The optical probe 12 is described in greater detail in the above-referenced Provisional U.S. Patent Application entitled “Disposable Tissue Probe Tip” and U.S. Patent entitled “Fiber Optic Light Mixer.”
A reference light signal was also provided by the probe connector 26. The reference light signal included a portion of the light from each of the LEDs, and had not been transmitted from the probe 12 before being collected. In the embodiment shown in FIG. 2, the reference light signal was collected by reference light signal send optical fibers 54, 56, 58 and 60, which extended respectively from each measurement light signal source LED 30, 32, 34, 36 to a light mixer/attenuator 62 formed by scattering material attached to a reference fiber fixturing ferrule 64. The reference signal send fibers 54, 56, 58, 60 were collected in the fixturing ferrule 64 at the scattering material along with a reference signal receive fiber 66. The reference light received from each LED was mixed at the mixer 62 and transmitted through the reference signal receive fiber 66. The end of the reference signal receive fiber 66 terminated in the probe connector 26 at a reference ferrule 68 which was adapted to mate with a socket in the connector 18 of the electronics package 14.
Since it was significantly attenuated when it was transmitted through the tissue, the intensity of the measurement light signal at the connector 26 was much less than the intensity of the non-attenuated reference light signal (e.g., about 1 million times less). In order to match the reference and measurement signal magnitudes to enable detection with a similar photo multiplier tube gain, the reference signal was attenuated at the mixer 62. The reference signal attenuation was obtained by reflectance mode positioning the reference signal send fibers 54, 56, 58, 60 equidistant from the centrally located reference signal receive fiber 66. The concentration of scattering material (such as titanium dioxide from Aldrich, Milwaukee, Wis.) within an optically clear epoxy substrate (such as EpoTech 301 from Epoxy Technology, Billerica, Mass.) could have been adjusted to provide the appropriate level of attenuation within the mixer 62. The probe connector 26 also preferably had a 14 pin electrical connector 72 and an optical fiber fixturing ferrule 74 for each of the LED's 30, 32, 34, 36, and 38, each of which were mounted in a PC board 76, along with connector 72. LED 38 was a calibration recognition signal LED connected to a calibration recognition send fiber 78. It is to be understood that the arrows on fibers 40, 42, 44, 46 were to indicate “to probe tip” while the arrows on fiber 48 were to indicate “from probe tip.”
A connector latch mechanism (not shown) latched the sample ferrule 52 and reference ferrule 68 of the probe connector 26 to the corresponding sockets (not shown) of the connector 18 in the electronics package 14. The latch connector mechanism is described in greater detail in U.S. Pat. No. 6,481,899 entitled “Optical Connector Latch Mechanism for Spectrophotometric Instrument.”
The reference light signal and measurement light signal (also referred to as a sample light signal) received at the connector 18 at spatially separated paths were collimated by lenses or other optics and directed to a shutter and path-shifting optics 80 (prior art FIG. 3). The shutter and path-shifting optics 80 selectively and alternately directed or folded the signals into a common path to the detector 20 (optical bench). One embodiment of the shutter and path-shifting optics 80 is illustrated in FIG. 3. As shown, a 30° stepper motor 87 drove opaque vane 84 and was controlled by the processor/controller 22, as indicated by arrow 86. The stepper motor 87 positioned the vane 84 to selectively block one of the reference light signal and measurement light signal, and to transmit the other of signals to the path-shifting optics 80. Arrow 88 indicates a collimated LED reference light path, while arrow 90 indicates a collimated measurement/sample light path (from the probe 12).
In the embodiment shown, the path shifting optics 80 included a 45° combining (beam splitting) reflecting member 92 in the measurement light path 94. This combining reflecting member 92 allowed a significant portion (e.g., 98–99%) of the measurement light signal to pass through the reflecting member 92 to the detector 20 (see FIG. 1) as indicated by arrow 96, with the remaining amount (e.g., 1–2%) being reflected away from the detector 20 (i.e., trapped, as indicated by arrow 98). A 45° reflecting member 100 in the reference light path 102 reflected the reference light signal onto the side of the combining reflecting member 92 opposite the side to which the measurement light signal was initially directed. A significant portion of the reference light signal would then pass through the combining reflecting member 92, while a smaller amount (e.g., 1–2%) would be reflected to the detector 20 (see FIG. 1) along the same optical path 96 as the measurement light signal. The measurement light signal and reference light signal were thereby directed or folded onto the same path 96 and directed to a common detector. In response to control signals from the processor/controller 22 (see FIG. 1), the stepper motor 87 would position the opaque vane 84 to block one of the reference light signal or the measurement light signal. The other of the reference light signal and the measurement light signal would then be transmitted to the detector 20. This optics configuration also reduced the intensity of the reference light signal so it would not saturate the PMTs of the detector 20.
Prior art FIG. 4 is an illustration of a detector 20 for use in the instrument 10 or electronics package 14 shown in prior art FIG. 1 and described above. An approximate 5 mm diameter collimated light beam indicated by arrow 104 (either from the reference or sample (measurement) light signal) was transmitted to the front surface of an 800 nm dichroic reflecting member 106 which was positioned 30° from an optical axis 108. Approximately 90% of the light having a wavelength greater than 780 nm was reflected to a first photomultiplier tube (PMT) sensor 110 which had an 800 nm bandpass filter (+/−10 nm at full-width, half-maximum (FWHM)) positioned in front of the PMT sensor 110.
Approximately 80% of the light having a wavelength shorter than 780 nm was transmitted through the 800 nm dichroic reflecting member 106 to the front surface of a 760 nm dichroic reflecting member 112 which was positioned 25° from the optical axis 108. Approximately 90% of the light having a wavelength greater than 740 nm was reflected to a second PMT sensor 114 which had a 760 nm bandpass filter (+/−10 nm FWHM) positioned in front of the PMT sensor 114. Approximately 80% of the light having a wavelength shorter than 740 nm was transmitted through the 760 nm dichroic reflecting member 112 to the front surface of a 720 nm dichroic reflecting member 116 which was positioned 30° from the optical axis 108. Approximately 90% of the light having a wavelength greater than 700 nm was reflected to the third PMT sensor 118 which had a 720 nm bandpass filter (+/−10 nm FWHM) positioned in front of the PMT sensor 118. Approximately 80% of the light having a wavelength shorter than 700 nm was transmitted through the 720 nm dichroic reflecting member 116 to the front surface of a 680 nm dichroic reflecting member 120 which was positioned 30° from the optical axis 108. Approximately 90% of the light having a wavelength greater than 660 nm is reflected to the fourth PMT sensor 122 which had a 680 nm bandpass filter (+/−10 nm FWHM) positioned in front of the PMT sensor 122. Approximately 80% of the light having a wavelength shorter than 660 nm was transmitted through the 680 nm dichroic reflecting member 120 to a detector block consisting of a 600 nm short pass filter (transmitted light from approximately 400 nm to 600 nm) positioned in front of a photo diode detector. This detector was used to measure the presence of ambient light and/or the calibration material recognition signal (530 nm LED emitter). The calibration material recognition signal and the manner by which it was used is described in U.S. Pat. No. 6,667,803 entitled “Calibration Mode Recognition And Calibration Algorithm For Spectrophotometric Instrument.”
During calibration procedures performed by the instrument, and for each of the PMTs used in connection with the calculation of the measurement (4 PMTs in the described embodiment), a baseline reading was established for both the measurement signal received from the probe (i.e., a baseline sample) and the reference signal (i.e., a baseline reference). These calibration measurement and reference baseline signals (for each PMT) were obtained through the use of the shutter and path-shifting optics 80 described above, and were stored in memory (not separately shown) and subsequently used in the measurement calculation algorithm.
Prior art FIG. 5 illustrates an optical probe 130 which was used in connection with the instrument shown in the above referenced U.S. Patent entitled “Tissue Chromophore Measurement System” and which included a light mixer 132. The probe 130 included an insert 134 for holding a number of optical fibers 136, 138, and 140, a housing 142 into which the insert 134 was mounted and a disposable elastomeric tip (not shown) which was releasably mounted to the housing 142. The optical fiber 136 terminated at a mixing fiber 144 and was coupled between the housing 142 and instrument within a cable housing 146. The illustrated embodiment of the probe 130 had four send fibers 136 through which light of different wavelengths from the instrument (provided by narrow bandwidth LEDs) was transmitted to the probe 130. The ends of the send fibers 136 were sealed in a ferrule 148. The light mixer 132 was a section of optical fiber 144 located between the fiber ferrule 148 and a tissue-facing surface 150 of the probe 130. The light mixer 132 accepted, on its input side, light from the individual send fibers 136. The light mixer 132 enhanced the homogeneity of the light emitted on its output side and transmitted to the tissue. Each wavelength of light was scattered over the whole cross-sectional area of the fiber 144 of the mixer 132, enabling each wavelength of light to travel through a similar volume of tissue.
As shown, the send fibers 136 were bent or formed to direct the ends at a 90° angle with respect to the tissue-facing surface 150. The different wavelengths of light emitted from the ends of the send fibers 136 were mixed within the fiber 144 of mixer 132 and thereby scattered throughout the surface area of the fiber 144 at the tissue-facing surface 150. As shown, a receive fiber 138 and a calibration recognition fiber 140 also had ends which terminated at the tissue-facing surface 150 of the probe 130. The receive fiber 138 collected light that traveled through the tissue being analyzed and transmitted the collected light to the instrument for processing. Light emitted from the calibration recognition fiber 140 was used by the instrument to control a calibration procedure.
Typical prior art instruments directed measurement light signals onto the tissue sample by bending the optical fibers in the probe to direct the light onto the tissue (see FIG. 5). The typical minimum recommended bend radius for an optical fiber is twenty times the fiber diameter, although this number may vary widely depending upon the type of optical fiber. Bending or shaping an optical fiber at less than the recommended minimum bend radii results in signal impairment or light signal loss, temperature sensitivity, and broken fibers. However, desirable spatial limitations in a probe are generally not suited to accommodate the minimum recommended bend radius of optical fibers. Generally, smaller sized probes are desirable for engaging smaller tissue sample areas and/or smaller test subjects, and are considered to be more comfortable and less intrusive for the test subject. As a result, prior art instruments were either large enough to accommodate the minimum recommended bend radius of the optical fibers, or produced lower quality light signals through over-bending of the optical fibers.
While the prior art structure for putting light at the surface of the tissue under study worked, high signal losses were encountered in the path between the LEDs and the tissue. Further, significant manufacturing effort and parts costs were incurred to make all of the optical paths required. Also, calibration procedures had to be repeated periodically to compensate for drift in the light source wavelength.